DES-induced pituitary adenomas are being characterized in terms of cell type(s), molecular forms of intracellular and secretal prolactin (PRL), and PRL-PRL cell interaction. Data show that this tumor will be useful as a model for studying PRL cell turnover as specifically related to the human disease. The source of the protein PIF in rat serum is apparently found in elements in the blood. A PRL ETA has been successfully developed. Human pituitary cells prepared from prolactinoma tissue have been implanted into F-344 rats using hollow fibers. Viable human pituitary cells are contained in these fibers up to approximately 60 days postimplantation.